Both vitamin D deficiency and skin cancers arising from epidermal keratinocytes are highly prevalent in Veterans. Although exposure to the ultraviolet (UV) wavelengths normally found in sunlight initiates vitamin D biosynthesis, the practice is not currently recommended since UV is also the principal cause of skin cancers. However, there remains uncertainty as to whether the relatively low doses sufficient for vitamin D production in the skin are associated with a meaningful risk of skin cancer in healthy individuals, and the Institute of Medicine's report on this topic identified resolving this issue as a major research need. Based on work in both our laboratory and those of others, there is evidence that UV-inducible mechanisms such as DNA repair might be directly coupled to vitamin D signaling. For example, we have observed that mice lacking the vitamin D receptor are prone to develop UV-induced epidermal tumors, and vitamin D and some of its metabolites induce DNA repair proteins. On the other hand, mice unable to synthesize the most biologically active form of vitamin D are not tumor-prone. However, it remains possible that one or more of the multiple other vitamin D metabolites does have anti-photocarcinogenic effects, including induction of nucleotide excision repair activity, the principal mechanism for removing UV-induced DNA lesions. The overall hypothesis of this research proposal is that vitamin D or its derivatives stimulate compensatory mechanisms to repair the collateral DNA damage associated with its own UV-mediated photosynthesis and thus minimizes photocarcinogenic effects. In Aim I, cultured keratinocytes and epidermal explants derived from CYP27B1-null mice will be used to compare the activities of major vitamin D3 metabolites for their effects on nucleotide excision repair. Both exogenous supplementation and chemical inhibition to modulate endogenous vitamin D metabolite levels will be employed, and expression of repair genes as well as repair activity and UV resistance will be assayed. In Aim II, both genomic and non-genomic mechanisms for vitamin D's effects on DNA repair will be investigated. Cultured keratinocytes and mouse epidermal explants will be used to assess the relevance of the vitamin D receptor in DNA repair, and the assembly and stability of nucleotide excision repair factors at lesions. Aim III will use mice treated with the most active vitamin D derivative and the broad cytochrome P450 inhibitor, ketoconazole, to assess whether supplementation or depletion of the major vitamin D3 metabolites protects animals from photocarcinogenesis, or predisposes them to it. Mutational spectra of tumors in the TP53 and XPC genes will also be analyzed to understand the pathways involved. These studies should make an important contribution to understanding the physiological and cellular mechanistic role of vitamin D in regulating DNA repair in skin and in suppressing UV photocarcinogenesis. The resulting fundamental biological insights may allow a rational basis to help guide health policy decisions regarding vitamin D in nutrition and skin cancer prevention for Veterans as well as the general population.